The present invention is directed to a frequency synthesizer for generating different frequencies in a microwave range, having a reference frequency branch and at least one phase-controlled control circuit that forms a phase locked loop (PLL) segment and, that for more than one phase-controlled control circuit, are cascaded according to a frequency divide and mix principle, and having an offset mixing stage for mixing a stable offset frequency with a reference frequency derived in the reference frequency branch from a stable crystal oscillator fundamental frequency, having a phase frequency detector for comparing an error signal to a frequency which is likewise derived from the stable fundamental frequency, and having a voltage-controlled oscillator to which an output voltage of the phase frequency detector is supplied via a loop low-pass filter as a control voltage and from which an output frequency as well as, potentially after being conducted through a frequency divider, an offset frequency for the phase-controlled control circuit of a following PLL segment are derived.
Frequency generators that can switch very quickly between different frequency channels and thereby supply extremely lownoise signals are required in many radio transmission systems, and particularly in radar equipment.
Synthesizers that work either direct or indirect, that is, with phase-locked loops, are employed for this purpose. The fundamentals and embodiments of such synthesizers are extensively described in the book by V. Manassewitsch, "Frequency Synthesizers Theory and Design", Second Edition, 1980, John Wiley & Sons, particularly pages 1-50. When prior art direct synthesizers, wherein, the output frequency is directly composed of various fundamental frequencies, that is, by mixing and multiplying, are employed for the above purpose, then the secondary lines and the noise far from the carrier are unsatisfactory. The article by D. Brewerton N. Urbaneta: "Synthesizer primer: Defining the elements of good design", in the periodical "Microwaves & RF", June 1984, Pages 79-86 and pages 124 and 125, makes proposals in conjunction with indirect synthesizers as to how quickly switching, phase-controlled control loops (high speed multi-loops) can be realized with good performance features. As may be derived from this latter reference, the phase noise of a phase-locked loop is dependent on the division ratio within the locked loop. The phase noise assumes a minimum value for a division ratio of one. When the center frequency error signal is produced by mixing, then care must be exercised so that the multiplication factors in the reference branch remain as low as possible since the noise is increased by N.sup.2 for every multiplication by the factor N. This proceeds particularly from the reference by V. Manassewitsch, particularly pages 119 through 124.